It is often necessary to protect an area against intrusion by giving an alarm when an intruder approaches the area, and it is preferred that the protection arrangement be not noticeable to the intruder, and hence less subject to being avoided.
Buried line sensors have been proposed for use when the area to be protected is out of doors. They have operated on various magnetic, electrical, and optical principles, and have utilized costly, fragile, and complex transducers easily damaged by careless handling or adverse elements.